The effect of reaction conditions and catalysts on the pyrolysis of polyethylene

Introduction: Pyrolysis of plastic is a green technology for converting plastic into fuel. This work studies the thermal and catalytic pyrolysis of polyethylene. Methods: In this work, polyethylene was pyrolyzed in a batch reactor at temperatures ranging from 500 to 600 °C. Acid oxides (Al2O3 and TiO2) and base oxides (MgO and CaO) were used as catalysts. The catalysts were characterized by XRD to confirm the materials used. Results: The results showed that the reaction temperature and reaction time affected the percentage of product fractions. A higher reaction temperature and time yielded a greater gas fraction, but hard conditions (600 °C and 3 h) produced a liquid fraction. The results also showed that thermal pyrolysis (for 3 h) yielded a greater liquid fraction and less gas fraction than that of catalytic pyrolysis (for 2 h and 3 h) at 600 °C. However, the use of oxide catalysts improved the quality of the liquid fraction compared with that of thermal pyrolysis, except for the TiO2 catalyst. Indeed, the results showed that the liquid oil fraction in catalytic pyrolysis exhibited a greater selectivity for hydrocarbons ranging from C7 to C20 than that in thermal pyrolysis. The base oxide catalysts (CaO and MgO) produced lighter hydrocarbons from C7 to C12, and the acid oxide catalyst (Al2O3) yielded a wider hydrocarbon distribution than that of the base oxide catalysts. Conclusion: Pyrolysis is a promising method for converting polyethylene plastic into fuel. The quality of the liquid fraction was improved by using oxide catalysts for pyrolysis. The hydrocarbon range of the liquid fraction can be tailored by using different oxide catalysts with lighter hydrocarbons (C7 – C12) for CaO and MgO catalysts and a wider hydrocarbon range (C7 – C20) for Al2O3. The liquid fraction from polyethylene pyrolysis in this study can be used in gasoline and diesel fuel.